Digital signaling has become an increasingly important feature of telephone services. Standard telephone service may be provided by a public switched telephone network (PSTN). Services provided by a PSTN may generally include the routing and transmission of telephone calls, as well as various supporting services such as call forwarding, display of a caller's name and number, and three-way calling, among others. At the edge of a PSTN, a telephone service provider may provide service to customers through basic telephone channels over copper conductors. In such channels, a voltage signal may be used to represent an analog signal, such as a scalar time-varying sound pressure level of a person's voice. The PSTN may band-limit such signals to a 3 kHz bandwidth, so that several such signals may be multiplexed for efficiency on the PSTN's links.
Digital information may be used in a variety of aspects of a telephone network. Customers of a local exchange carrier may transmit digital data over basic telephone channels to a subscriber's site. For example, even basic band-limited channels may be used for modem signals, in which the voltage signal uses amplitude-shift, frequency-shift, or phase-shift keying (ASK, FSK, or PSK), or combinations thereof, to convey a stream of binary data. PSTN providers may also provide wider-bandwidth channels for subscribers, with greater capacities for voice and data signals. Examples of such higher-capacity services include integrated services digital network (ISDN) lines, digital subscriber loop (DSL) services, T-1 lines, and T-3 lines, among others. A general tool for identifying users of these services is the telephone number, which may identify individual subscriber ports assigned to a carrier's customers. Telephone numbers may have forms such as “1-408-555-4000,” and may be assigned in conformance with various standards and conventions, such as the E. 164 standard set by the International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T).
Independently of their use at the edges of a PSTN by customers, digital signals may also be used at the heart of a PSTN network. For example, a local exchange carrier may digitize incoming telephone signals so that the signals may be more readily multiplexed and switched through the telephone network. The digital signals also allow use of fiber-optic networks, using on-off keying (OOK) to efficiently convey many multiplexed signals over long distances. The PSTN may transmit digital signals according to the Common Channel Signaling System No. 7 (i.e., SS7 or C7), a global telecommunications standard from the ITU-T. SS7 broadly establishes protocols for a PSTN to transmit signals over a digital network to set up, route, and control wireless (cellular) and wireline calls.
Digital signals are also used for telephony by individuals who may digitize voice signals for transmission over the Internet, with our without the use of a PSTN. Colloquially known as voice over internet protocol (VoIP) connections, these connections may make only incidental use of a PSTN, as in the case of two people who communicate with each other over the Internet using headsets and their personal computers. VoIP connections may also partly interact with a PSTN, as in the case where one person uses a headset and a personal computer to initiate a telephone call to a PSTN subscriber.
Yet another form of digital telephony is the use of localized digital telephone networks, which may or may not be connected to a PSTN. One form of a localized digital telephone network is an IP telephone system, which uses internet-related protocols such as the transmission control protocol (TCP) and the user datagram protocol (UDP) to convey voice signals among telephones connected to a digital network. The technology of an IP telephone system may also be called VoIP. Other forms of localized digital telephone network may use other, non-internet-related, protocols for communicating digitized voice signals.
Among the challenges faced by designers of new tools for digital telephony is the simplification of products, so that users may quickly and easily install, initiate, and use the products. This simplification is especially important to designers of systems intended for use by customers outside of the telecommunications industry. Consumers and small-business owners may benefit from careful design that makes their systems user friendly and easy to install.
For example, a small business may have several PSTN lines for use by the business's telephones. If the business purchases a localized digital telephone network, an initial challenge may be to connect the digital telephone network to the PSTN through the existing telephone lines. One task may be to determine which of several telephone numbers correspond to which of several port numbers used internally by the localized digital telephone network. Tools and procedures that simplify the interfacing of a localized digital telephone network to a PSTN may be valuable to users faced with the task of installing such a network.